netdev - [PATCH 1/6] bpf, docs: Add a setion to explain the basic instruction encoding

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Message-Id: <20220103183556.41040-2-hch@lst.de>
Date:   Mon,  3 Jan 2022 19:35:51 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jonathan Corbet <corbet@....net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, linux-doc@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: [PATCH 1/6] bpf, docs: Add a setion to explain the basic instruction encoding

The eBPF instruction set document does not currently document the basic
instruction encoding.  Add a section to do that.

Signed-off-by: Christoph Hellwig <hch@....de>
---
 Documentation/bpf/instruction-set.rst | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index 1af51143ff9f6..80f42984b5942 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -19,8 +19,22 @@ The eBPF calling convention is defined as:
 R0 - R5 are scratch registers and eBPF programs needs to spill/fill them if
 necessary across calls.
 
+Instruction encoding
+====================
+
+eBPF uses 64-bit instructions with the following encoding:
+
+ =============  =======  ===============  ====================  ============
+ 32 bits (MSB)  16 bits  4 bits           4 bits                8 bits (LSB)
+ =============  =======  ===============  ====================  ============
+ immediate      offset   source register  destination register  opcode
+ =============  =======  ===============  ====================  ============
+
+Note that most instructions do not use all of the fields.
+Unused fields shall be cleared to zero.
+
 Instruction classes
-===================
+-------------------
 
 The three LSB bits of the 'opcode' field store the instruction class:
 
-- 
2.30.2